It is already known to permanently anchor oral prostheses in the jawbone with the aid of screw-shaped securing elements, so-called fixtures, made of a biocompatible material, preferably pure titanium. The method which has been found to give the highest degree of anchoring stability and which has been used clinically with good results for over 20 years is the so-called osseointegration method developed by Professor Per-Ingvar Br.ang.nemark and co-workers. The method is based on a very exact and atraumatic technique for inserting the fixtures in such a way that direct contact, that is exact fitting without an intermediate connective tissue, is achieved between a fixture and a bone tissue. Such direct contact between the fixture and the bone tissue gives the best preconditions for a truly permanent anchoring of a dental prosthesis.
It is also already known to permanently anchor extraoral prostheses. Since 1977, the otology clinic at Sahlgrenska Hospital in Gothenburg has, in collaboration with the Institute of Applied Biotechnology and Nobelpharma AB, been carrying out research on skin-penetrating titanium implants.
The first clinical application was a so-called bone-anchored hearing aid for patients who, for various reasons, cannot use a conventional hearing aid. A screw-shaped titanium implant is introduced by surgery and is allowed to osseointegrate in the bone behind the ear. In a second operation, the titanium screw is exposed and a skin-penetrating element made of titanium is attached. A hearing aid can be mounted on the latter, and via vibrations in the bone, it stimulates the hair cells of the cochlea, see SE-A-431,705.
Another application area for tissue-anchored titanium implants with skin penetration is the attachment of facial prostheses. Nearly seventy patients with defects of the external ear have been provided with prostheses (see Tjellstrom A, Yontchev E, Lindstrom J, Br.ang.nemark P-I. Five years experience with bone-anchored auricular prostheses. Otolaryngology-Head and Neck Surgery 1985,93, No 3). See also SE-A-450,810 which describes an arrangement for attaching a prosthesis, in particular an auricular prosthesis, in a number of securing elements, implanted in the body tissue, with the aid of a splint.
It is also already known to permanently anchor ocular prostheses (orbital prostheses) in the cranium. For some ten years, patients with orbital defects, for example following tumor surgery, have been treated with good results using tissue-integrated and skin-penetrating implants. See, for example, Jacobsson M, Tjellstrom A, Thomsen P, Albrektsson T: Integration of titanium implants in irradiated bone tissue; Annals of Otolaryngology, 1986.
After the patient has had the orbit and the floor of the orbit removed, the orbital prosthesis is installed on, for example, three osseointegrated securing elements. The orbital prosthesis can be manufactured with extremely thin edges which permit facial movements without the defect being revealed. This method represents a distinct improvement for the patient, when compared with previous orbital prostheses which were secured on glass eyes.
The implants which have hitherto been used when treating patients with facial prostheses have consisted of a securing element (fixture) anchored in the cranium and a skin-penetrating part which is attached to the securing element, the titanium screw, in a second operation. The securing element (fixture) can consist, for example, of a so-called flange fixture according to Swedish design model No. 42 382.
The skin-penetrating elements (spacing members) which have hitherto been used have been essentially sleeve-shaped and have formed an extension of the securing element in the longitudinal direction thereof. This has caused difficulties in installing the prosthesis, on account of the fact that the accessibility is limited within the orbit. On account of the length of the spacing member there is often an axial/radial lack of space, and this can also be made worse by the non-parallelism of the implants. It is not only the actual installation of the prosthesis, but also the taking of an impression and the fitting of the prosthesis which are made more difficult because of the lack of space arising in the case of the previously used spacing members. The installation of an orbital prosthesis is particularly complicated since the bone quality is often poorer within the orbital area. During installation, it is therefore generally desired to place the securing elements in the radial direction within the essentially circular cavity formed by the orbit, in order to achieve the best bone anchorage. However, from the prosthetics point of view, this is not expedient. For prosthetic work it is instead desirable to have an axial direction of attachment (in the direction of the eye).
It is also important that the spacing members should be designed in such a way that they do not project too far in the axial direction, since this encroaches upon the desired position for the prosthesis. In addition, in order to achieve a good transition between the outer contour of the prosthesis and the face, the prosthesis should, as far as possible, lie recessed within the orbital area.